There are two rules in science that every researcher soon learns: what you don’t see is as important as what you do see and a paradox means that you are on the edge of learning something amazing. In today’s adventure, Peter and Mary find this out for themselves as they see what darkness lurks in the hearts of galaxies!
Sharing a sunset with friends is always good. As the sun paints the sky with vivid shades of orange, red, and purple (and a flash of green), you can discuss the important stuff, such as what you like best and where you want to live when you get older. But the best part is just being there with your friends, sharing something unique and special. And that’s what Peter and Mary were doing on a particularly clear summer night – just hanging out and being friends.
Of course, for them, some of the answers were already known. They both liked science best of all, and enjoyed doing experiments whenever they could. And they didn’t really care where they lived as long as it was someplace where they could do experiments, though Mary really preferred Houston simply because that was where all of the other astronauts lived. But there were still some questions that they hadn’t answered. And Mary had one for Peter.
“So why did you mother say we should be out here tonight? She isn’t trying to get you out of the house so she can run more simulations, is she?” Peter’s mother was an astronomer who sometimes needed more peace and quiet than a house with a young son could provide, so she would ask Peter to run errands or do experiments outside while she concentrated.
“No, she just said that it was time that I learned some of the deepest, darkest secrets of the universe and told me to be here after sunset. I wonder what we’ll learn?” he mused.
“You’ll discover that just as soon as we get some basics down,” said a cheerful voice behind them; turning around, they saw Peter’s mother holding three small bags. “Come and grab these binoculars!”
As the two each took a bag and opened it to reveal a good but inexpensive pair of binoculars, Peter’s mother continued her explanation.
“We’re going to look into the night sky and see one of the most amazing things ever described,” she said. “But before we do, we need to figure out how to make sure that we are all looking at the same thing. Any ideas on how we could do that?”
“You keep muttering under your breath about ‘right ascension’ and ‘declination’ when you are working on a problem,” Peter replied.
“But I don’t know what those are!” Mary objected.
“Well, neither do I,” Peter admitted. “But it is what she says.”
“When we are in the lab or using a big, expensive telescope, we do use those terms,” Peter’s mother said. “They are a mathematical grid that helps us locate things in the sky, much as latitude and longitude help on the ground. But we don’t drive around using latitude and longitude. Instead, we use street signs and relative directions like ‘turn left after the next block’.”
“So what are the street signs in the sky?” asked Mary.
“The constellations,” replied Peter’s mother. “They tell us which part of the sky to look in. For example, the nearest star to the Sun is…”
“Alpha Centauri!” Peter interrupted.
“Close but no cigar,” his mother said. “It is actually Proxima Centauri; the name ‘Proxima’ means ‘nearest’ and ‘Centauri’ means that it is found in the Centaurus constellation. But Proxima is too dim to be seen with the naked eye and is right next to Alpha, astronomically speaking, so most non-scientists think that they are the same.”
“Why do we call it Alpha Centauri?” Mary asked.
“Ah, that’s like a street number in our sky map,” Peter’s mother replied. “The stars are given a designation based on their apparent brightness – how bright they look from Earth. The brightest star in a constellation is ‘alpha’, the next-brightest is ‘beta’, then ‘gamma’ and so on. Many of the brightest stars also have a name in Arabic -”
“Like Betelgeuse!” Mary said.
“Right, the good old ‘shoulder of the giant’. And we’ve got ‘the follower’ Aldebaran in Taurus or ‘the tail of the hen’ Deneb in Cygnus. But you can’t drive with a street map in your face and you can’t find stars at night using those names until you’ve got a lot more experience. So tonight, we’ll use the old astronomer standby of ‘face East and make a fist’.”
“Huh?” said Peter. “What good will that do?”
“Well, let’s start by facing East,” his mother replied. Humoring her, the two obediently turned to face the opposite direction from where the Sun had gone down.
“Now spread you hand wide and sweep it from East to directly overhead to West,” she commanded. As they did so, Peter’s mother explained, “What you’ve just done is sweep out half of the ecliptic plane; that’s the band where most of the stuff in the Solar System lives. Most of the planets and asteroids and other junk in our Solar System will be found in the band you just described with your hand. So if we ever go looking for planets, that’s where we can look.”
“Neat!” Mary said. “So we can see Jupiter?”
“If it was up tonight, we could,” Peter’s mother said. “But it won’t be up until well after you two go to bed, which is a shame because you could see the moon of Jupiter with your binoculars. We’ll have to try it some night when Jupiter is up earlier. But for tonight, what I want you to to is face East and count three fists to the North and four fists up. When you put a fist at arm’s length, it is always about ten degrees, no matter who or how old you are, so fists are an astronomer’s favorite quick and dirty measuring method. When you get to the position, look at it and tell me what you see.”
“That’s just old Orion’s sword,” Peter said. “There’s nothing special there.”
“I wouldn’t be so sure,” his mother replied. “Look at it again, only use your binoculars.”
The two looked through their binoculars at the sword hanging from Orion’s belt and let out gasps of wonder.
“It’s full of stars!” Mary exclaimed.
“Even better,” Peter’s mother said. “It is full of baby stars and planets; that is a stellar nursery where new solar systems are being born.”
“Wow!” Peter said. “Is the whole sky like this?”
“That’s exactly why I asked you to come out here tonight,” said his mother. “When I was your age, your grandfather brought me out to a field and showed me what you’ve just seen. And then he made a bet with me. He bet me $10 that I couldn’t find a part of the sky without stars.”
“That’s easy,” Mary said. “There’s an empty patch over there!”
“Look at it through the binoculars,” Peter’s mother said.
“Whoa! It has stars!”
“Yes, that’s right. So here’s my challenge to you two – can you fins any part of the sky that has no stars in it when seen through the binoculars?”
“Sure we can!” Peter said.
“I don’t know,” Mary replied. “I thought my patch was dark but it really wasn’t.”
Well, there is only one way to know for sure,” Peter’s mother said. “Do the experiment!”
What do you think will happen? Do the experiment yourself!
For twenty minutes, Peter and Mary searched the sky looking for patches without stars. Time after time they’d get excited about a seemingly empty area only to see it fill with stars when they looked at it through the binoculars. They tried down near the horizon and up near the zenith overhead, but every area had stars. Finally, the two gave in and admitted that the entire sky was filled with stars.
“No matter where we look, there are stars in the sky,” Peter said. “But why don’t we see them?”
“For the same reason that you don’t see a candle from a mile away,” his mother explained. “Stars are just big light sources in the sky. And the farther away they are, the dimmer they get. You can see them with the binoculars because they gather more light and make dim things visible. Telescopes do the same thing; all binoculars are is two telescopes strapped together. As a matter of fact, when the Hubble telescope stared at one place in the sky for five days, it discovered over a thousand galaxies in a patch of the sky that you could cover with your thumb. That means billions of stars and trillions of planets, all out there in the deepness of space.”
“Wow!” Mary said. “But if there are all of those stars out there, then why is the night sky dark?”
Peter’s mother chortled happily.
“You have just discovered Olber’s Paradox!” she said. “Back in 1823, an astronomer by the name of Olber made famous a problem that had puzzled astronomers since Kepler’s day: ‘if the universe is infinite and there are stars wherever we look, then why is the night sky dark?'”
“So what is the answer?” demanded Peter.
“I’ve given you enough hints to know the answer,” his mother replied.
“You said “if the universe is infinite-‘” Mary started.
“That’s it!” Peter exclaimed. “The universe isn’t infinite!”
“Got it in one!” his mother said. “The solution to the paradox is that the universe isn’t infinite. As a matter of fact, it is only about 13.8 billion years old – remember that infinite implies age as well as width – and so there isn’t enough space for an infinite number of stars nor has there been enough time an infinite number of stars to have formed.”
“Gosh!” Mary said.
“Gosh indeed,” replied Peter’s mother. “And now that we’ve solved the paradox, let’s just look at the stars.”
Turning their binoculars skyward, that’s what they did.